The present invention relates to the production of printed images composed of a pattern of dots produced by the application of individual ink drops to a printing medium.
In the printing art, there exist printers which operate according to a variety of techniques, many of which fall in the category of printers that form images upon a printing medium by depositing rows of ink dots in a desired pattern. Printers of this type generally include a print head provided with an array of nozzles for ejecting ink droplets toward the printing medium. During relative movement between the print head and the printing medium, referred to herein as scanning, each nozzle traverses a line, or pixel row, on the printing medium and deposits ink drops at desired locations, or pixel areas, along that row. These relative movements are typically referred to as scanning. Bands of ink dots are formed on the print medium by effecting a relative displacement between the print head and the print medium, referred to herein as indexing, in a direction generally perpendicular to the direction of relative movement during each printing pass.
The inks employed in such printers include aqueous inks which are liquid at room temperature and phase change inks which are solid at room temperature and which change to a liquid state upon being heated. Aqueous inks contain a pigment dissolved in a solvent, the solvent evaporating after delivery of each ink drop to the printing medium, leaving behind pigment which has penetrated into the printing medium. Phase change inks are heated to remain in the liquid state until striking the printing medium, whereupon each drop of ink solidifies by cooling.
Among the techniques which have been developed for performing printing are interlace techniques in which the spacing between nozzles is such that alternate pixel rows are printed during one printing pass and intervening pixel rows are printed during a subsequent printing pass. Interlacing patterns can be provided to minimize printing defects, such as the horizontal banding effect which results from cross-talk between nozzles, as well as the seaming effect which is caused by variations in the amplitude of the relative movements between printing passes. In addition, particularly in the case of phase change inks, interlacing allows each pixel row to be printed at a time when both adjoining pixel rows have not yet been printed or have both been printed, thereby creating a condition of thermal symmetry, which improves the appearance of the resulting printed image. In addition, for a given image resolution, expressed in terms of dots per inch (dpi), interlaced printing of the type described above makes possible an adequate spacing between nozzles which simplifies print head manufacture.
Printers of the type described above have also been developed for producing multicolor images by applying ink dots in a variety of colors, such printers generally being equipped to perform printing with black ink and with a set of primary color inks such as cyan, magenta and yellow inks. It has been found that by applying dots of inks of different colors atop one another, or immediately adjacent one another, a wide range of colors can be reproduced including, in addition to the primary colors mentioned above, red, green and blue, which are secondary colors formed from various combinations of two of the primary colors mentioned above.
It has further been proposed to enlarge the color palette available for printed images by dividing the printing medium surface into an array of super pixels each constituting a generally square area of four or more elemental pixels. Each super pixel represents one picture element and can be printed with a selected combination of ink dots. As a result, a wide variety of colors and gray scale levels can be produced at each super pixel. Arrangements of this type are disclosed, for example, in U.S. Pat. Nos. 4,967,203, Doan et al, and 5,111,302, Chan et al.
In the apparatus disclosed in the patent to Doan et al, alternate super pixels in each super pixel row and column are printed during one print head pass, and intervening super pixels are printed during the next succeeding print head pass.
While the Chan et al patent mentions the applicability of the disclosed technique to ink jet printers, it only provides a detailed description of a printing operation performed with a laser printer, and provides no information relating to the pattern or sequence in which super pixels are printed. The Chan et al patent does describe the possibility of varying the diameter of the ink dot formed at each elemental pixel in order to provide further gray scale levels in the printed image.
To date, no technique for forming a printed image of super pixels while retaining the advantages of true interlace scanning have been proposed.
In addition, interlacing schemes which have been previously proposed are effective to reduce printing defects. Nevertheless, they can give rise to printing defects when even small errors exist in the print medium indexing movements. Such defects include reflectance variations in the printed image and a high frequency seaming effect.